A guide used to constitute an amplifier or a laser, for example an optical fiber or a planar guide, traditionally comprises an amplifier medium including:                a core in a matrix of a transparent material incorporating active doping elements which are the source of the amplification phenomenon, such as metallic ions of rare earths such as erbium,        a cladding for guiding the luminous power which for the most part is transmitted by the core.        
Thus, the U.S. Pat. No. 5,717,517 provides information of an amplifying optical fiber comprising an amplifier medium including:                a core in a main matrix of a transparent material (glass or polymer) containing crystalline particles, each particle being formed of a submicronic matrix (forsterite, YAG, Ca2SiO4, or even LiGaO2, LiAlO2) distinct from the main matrix and doped by an active metallic ion (V3+ or Cr4+),        a guiding cladding surrounding the core.        
However, this fiber exhibits significant losses via diffusion of the light, thus limiting its optical performances.